1. Field of the Invention
The present invention relates to a disk wheel for an automobile, and more particularly, to an improved disk wheel in which satisfactory durability of a disk/rim welded portion is obtained in a dynamic radial fatigue test.
2. Description of Related Art
As shown in FIG. 12, there is a known disk wheel made of plate material for an automobile in which a disk 1 is fitted in an inner periphery of a rim 2, the disk and the rim being welded and connected to each other at a welded portion designated by reference numeral 3.
A portion A of FIG. 12 is shown in FIG. 13 on an enlarged scale. In this type of disk wheel, as shown in FIG. 13, a flange at a lower portion 4 of the disk 1 formed by a plastic working method comes to a tapered shape in such a manner that the diameter of the disk at the distal end (the lower end in the drawing) is larger than that at the proximal portion 6.
Consequently, when the disk 1 is fitted in the rim 2, as shown in FIG. 13, a radial clearance 7 is defined between the outer periphery of the proximal portion 6 of the flange portion 4 of the disk 1 and the inner periphery of the rim 2.
Under such a condition, when a dynamic radial fatigue test is carried out for the disk wheel, stress concentration is caused at the welded portion 3 so that the fatigue limit life at this portion is severely shortened. As a result, there occurs a crack in the welded portion 3.
In order to prevent the occurrence of the crack in the welded portion 3 in the dynamic radial fatigue test so as to prolong the fatigue limit life, it is necessary to take measures for closely fitting the outer periphery of the flange portion 4 of the disk 1 in the inner periphery of the rim 2 so that the clearance 7 therebetween is minimized.
Concretely, any of the following measures (1) to (3) has been performed for solving the above problem.
(1) A fitting interference of the disk 1 with respect to the rim 2 is increased to fit more closely. PA1 (2) In order to minimize an amount 8 of taper of the flange portion 4 of the disk 1 illustrated in FIG. 14, in the step of drawing the flange portion 4, a clearance between dies is reduced as much as possible. PA1 (3) When the disk 1 is formed by drawing, as shown in FIG. 15, the disk 1 is held between an annular drawing ring 25 and a molding punch 22, and it is strongly pressed by a molding die 23 in a direction of an arrow 10, thereby forming an arc-shape indicated by numeral 9 over the whole circumference of the disk 1. A dimension of the drawing ring 25 is so determined that an outer diameter of the disk 1 becomes a desired value.
In the measure (1) of the prior art, when the fitting interference is increased in order to establish sufficiently close contact between the disk 1 and the rim 2, a strong fitting force is required when the disk 1 is fitted in the rim 2. Accordingly, there occurs a scratch during fitting of the disk in the rim, which impairs a value of the disk wheel as a product. Also, due to the strong fitting force, the disk 1 and the rim 2 are sometimes deformed so as to lower an accuracy of the disk wheel.
In the measure (2) of the prior art, seizure occurs between the dies during forming the flange portion 4 because the clearance between the dies is reduced, so that there occur a lot of dimensional faults of the disk 1 or loss of much labor resulting from tedious adjustment of the dies. As a result, the productivity becomes worse.
In the measure (3) of the prior art, a degree of deformability of a member to be worked in correspondence to the inner periphery of the drawing ring 25 changes due to variation of forming conditions or variation in shut-height during processing of the member, or variation in thickness of the member to be worked. It is thus difficult to stabilize the dimension of the member to be worked. As a result, the durability of the product is liable to be unstable. Some of the products unfavorably have the short durability.